1. Field of the Invention
This invention relates to the operation of multiple electronic article surveillance (EAS) systems, and more particularly the wireless synchronization of EAS systems operating in the same vicinity of each other.
2. Description of the Related Art
EAS systems in close proximity often must be carefully synchronized to avoid adverse interactions. There are several different levels of synchronization possible. The transmitter's carrier oscillator can be synchronized, or the transmitter's modulating waveform can be synchronized. In more complex systems, such as those sold by Sensormatic Electronics Corporation under the trademark ULTRA*MAX, the transmitter configuration sequence can be synchronized between multiple systems.
U.S. Pat. No. 6,201,469, issued Mar. 13, 2001, by Sensormatic Electronics Corporation, covers synchronization of the transmitter configuration sequence. Synchronizing the transmitter sequence is important for EAS systems that are in very close proximity to each other such that their interrogation zones overlap. As disclosed in that application, the transmit burst timing is tied directly to the power line zero crossing function, for which the phase is manually adjusted.
There is a need for synchronizing the transmit carrier's modulating waveform for EAS systems in close proximity, even if their interrogation zones are not overlapping. In swept RF systems this means synchronizing the sweeping function between multiple transmitters. In pulsed systems, such as ULTRA*MAX, this means synchronizing the transmitter pulse function between multiple systems.
Pulsed EAS systems positioned within hundreds of feet of one another must have their transmit burst timing precisely aligned or the transmitters will interfere with one another's receivers, decreasing sensitivity or causing false alarms. In prior systems this has been accomplished by using the three phases of the power line for synchronization. Each system is plugged into the 60 (or 50) hertz power system, which is divided into three phases. Each phase is a sinusoidal function nominally offset from one another by 1/180 of a second (or 1/150 of a second for 50 hertz systems) apart. The zero crossing of the power line is used as a timing reference, assuming that this 1/180 second separation is correct. However, due to variations of loading conditions across the three phases of the power line, often they are not exactly spaced 1/180 seconds apart. This causes the systems to interfere with each other, which in turn causes a service call to local technicians. The technicians must come and manually adjust the timing of the systems. If loading conditions on the power lines change, the process repeats itself at great expense to the company.
Another problem with using the power line as a timing reference is that the power line is not necessarily sufficiently stable. In particular, the zero crossing has a significant amount of phase noise. This phase noise is translated directly to timing jitter on the system transmitters. Since the phase noise on the three line phases may not be correlated, the jitter experienced by multiple systems compounds the problem.
In swept RF systems whose interrogation zones overlap, interference of the two transmit signals can cause decreased performance. In the worst case, one transmitter may be sweeping low while the other is high, and visa versa. The envelope of the two transmitters (i.e., the carrier's modulating function) must be synchronized for best performance.
Synchronization of adjacent EAS systems can be accomplished by hardwiring the systems so that timing of each EAS system can be precisely controlled. Hardwiring of adjacent EAS systems is not always feasible or cost effective. Manual adjustment coupled to and power line zero crossings include the limitations described hereinabove. A wireless, automatic method of synchronizing the transmit carrier's modulating waveform for an EAS system is needed.